Half-sleeved and sleeveless plastic piston pumps

ABSTRACT

In one aspect, a brake pump assembly with a hydraulic block and a pumping element including a polymer piston that is received in a piston bore. A circumference of the polymer piston defines a high-pressure seal slideably engaging adjacent structure. The polymer piston may include a peripheral sealing lip projecting primarily longitudinally from the polymer piston for sealing engagement with the piston bore. In another aspect, a brake pump assembly with a hydraulic block, a straight bore polymer piston, a peripheral sealing lip projecting from a proximal portion of the polymer piston, and a elastomeric seal mounted to a distal portion of the polymer piston, where the peripheral sealing lip selectively acts to depressurize fluid accumulating between the peripheral sealing lip and elastomeric seal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 11/623,077, filed on Jan. 13, 2007, now U.S. Pat. No. 7,823,982the entirety of which is incorporated herein by reference.

BACKGROUND

The disclosure is directed to hydraulic piston pump elements for use ina brake pump assembly and, more particularly, to a stepped boreconfiguration, or may be a straight bore configuration, or may be eitherconfiguration piston pump elements. In brake systems which utilizeanti-lock brake systems (“ABS”), traction control systems (“TCS”),electronic stability control systems (“ESC”), and/or other controlledbraking systems to generate controlled braking events, it may be desiredto modulate pressure inside the brake system with a brake pump assemblyto enable controlled braking events.

Brake pump assembly suppliers are continually challenged to providenext-generation brake pump assemblies at a lower unit cost whilemaintaining or increasing performance and durability standards. Brakepump assembly designs have focused on a wide variety of strategies toaccomplish the lower unit cost objective, including but not limited to asimplified assembly sequence strategy, a reduced number of componentsstrategy, etc. Multiple assembly variations typically add time andexpense to the assembly sequence. However, eliminating components bycombining functions in a multiple assembly configuration may offset suchdisadvantages and provide another useful strategy for achieving a lowerunit cost objective.

Accordingly there is a need for a brake pump assembly having a lowerunit cost which meets or exceeds the performance and durabilitystandards of currently produced brake pump assemblies.

SUMMARY

Brake pump assemblies produced in accordance with certain aspects of thedisclosure provide lower unit costs without significantly detractingfrom performance and durability standards.

In one aspect, the disclosure relates to a brake pump assembly with ahydraulic block and a pumping element including a polymer piston that isreceived in a piston bore. A circumference of the polymer piston definesa high-pressure seal slideably engaging adjacent structure. The pumpingelement may include a half-sleeve, where the high-pressure seal is aninterface surface between a proximal portion of the polymer piston andthe half-sleeve, or the high-pressure seal may be an interface surfacebetween a proximal portion of the polymer piston and the piston bore.The polymer piston may include a peripheral sealing lip projectingprimarily longitudinally from the polymer piston for sealing engagementwith the piston bore.

In another aspect, the disclosure relates to a brake pump assembly witha hydraulic block, a generally cylindrical polymer piston slideably andsealingly engaging a piston bore, and a pump bore cap, where theelements define a sleeveless fluid pumping element in a motor vehiclebrake pump assembly. The polymer piston may include a unitary andintegral peripheral sealing lip projecting primarily longitudinally fromthe polymer piston to define a directional pressure seal with the pistonbore.

In another aspect, the disclosure relates to a brake pump assembly witha hydraulic block, a straight bore polymer piston, a peripheral sealinglip projecting from a proximal portion of the polymer piston, and aelastomeric seal mounted to a distal portion of the polymer piston,where the peripheral sealing lip selectively acts to depressurize fluidaccumulating between the peripheral sealing lip and elastomeric seal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a vehicular brake system according toaspects of this disclosure;

FIG. 2 is a cross-section view of one aspect of a disclosed pumpingelement in the form of a straight bore configuration;

FIG. 3 is a cross-section view of an another aspect of a disclosedpumping element in the form of a stepped-bore configuration;

FIG. 4 is a graph of brake fluid output as a function of instantaneouspump motor shaft angle for a horizontally opposed configuration, showingthe output of three assemblies over three differently labeled cycles;

FIG. 5 is a cut-away front perspective view of a horizontally opposedhydraulic block configuration according to aspects of the disclosure;

FIG. 6 is a cut-away front perspective view of a radial-style hydraulicblock configuration;

FIG. 7 is a cross-section perspective view of one aspect of a brake pumpassembly having the hydraulic block configuration shown in FIG. 5 andincluding six pumping elements driven by a rotatable eccentric assembly;

FIG. 8 is a front perspective view of a press-fit eccentric assembly forthe device of FIG. 7;

FIG. 8A is a perspective view of an eccentric needle bearing elementshown in FIG. 8;

FIG. 8B is a cut-away perspective view of the rotatable eccentricassembly shown in FIG. 8;

FIG. 9 is a cross section perspective view of a pumping element having a“half-sleeve” design with a polymer piston body;

FIG. 10 is a cross section perspective view of a pumping element havinga sleeveless design with a polymer piston body;

FIG. 11 is cut-away perspective view of an electro-hydraulic controlunit or “EHCU” having an embedded wheel pressure sensor;

FIG. 12A is a perspective view of an EHCU having an embedded wheelpressure sensor and sensor signal path, with the electronic control unitportion omitted;

FIG. 12B is a cut-away perspective view of the EHCU of FIG. 12A showingan interface between a hydraulic control unit portion and an electroniccontrol unit portion;

FIG. 13 is a cross section perspective view of an aspect of a brake pumpassembly similar to that shown in FIG. 7, but having pumping elementscoupled to a rotatable eccentric assembly by an external pump pistonretainer;

FIG. 14 is a cross section perspective view of the pumping element shownin FIG. 9 installed within a hydraulic block, illustrating pressureseals made of a polymeric material in combination with a “half-sleeve”pumping element design;

FIG. 15 is a cross section perspective view of the pumping element shownin FIG. 10 installed within a hydraulic block, illustrating pressureseals made of a polymeric material in combination with a sleevelesspumping element design;

FIG. 16 is a cross section perspective view of a pumping elementinstalled within a hydraulic block, illustrating a flexible, directionalpressure seal provided as a high-pressure seal in a polymer pistonelement;

FIG. 17 is a cross section perspective view of a pumping element similarto that shown in FIG. 10 installed within a hydraulic block,illustrating a flexible, directional pressure seal provided as anadditional low-pressure seal in a polymer piston element.

DETAILED DESCRIPTION

As shown in FIG. 1, a basic motor vehicle brake system, generallydesignated 10, includes a master cylinder 12 having a reservoir 14,wherein the master cylinder 12 is operatively coupled to a brake pedal(not shown). The master cylinder 12 is configured such that when thedriver presses the brake pedal brake fluid in the master cylinder 12 ispressurized and delivered to a pair of ports 16, 18, with a primarybrake line 20 coupled to and extending from the port 16 and a secondarybrake line 22 coupled to and extending from the other port 18. A frontbrake subsystem, generally designated 23, may be coupled to the primarybrake line 20 and include a pair of normally open apply valves 24 and apair of normally closed release valves 26. The subsystem furtherincludes a pair of apply check valves 28, with each apply check valve 28being in parallel with the associated apply valve 24, and a pair ofrelease check valves 30, with each release check valve 30 being inparallel with the associated release valve 26.

Fluid passing through each apply valve 24 flows to an associated wheelbrake subsystem, generally designated 31, for braking either the rightfront (“RF”) or left front (“LF”) wheel, or the right rear (“RR”) orleft rear (“LR”) wheel, as further described below. The pressurizedfluid causes a caliper 32 in the wheel brake subsystem 31 to compressand thereby cause a brake pad or pads to engage a rotor 34 to brake theassociated wheel in a well-known manner.

The outlets of the release valves 26 are in fluid communication with anaccumulator 36 such that fluid is collected in the accumulator 36 as itis released from the wheel brake subsystems 31. The accumulator 36 is influid communication with a pump, generally designated 40, via a pumpinlet line 42. The pump assembly 40 may include six pumping elements 40a, 40 b, 40 c, 40 d, 40 e, and 40 f, three of which 40 a, 40 b, and 40 cmay serve the front brake subsystem 23. A motor 46 is operativelycoupled to each pumping element 40 a, 40 b, 40 c, 40 d, 40 e, and 40 fto reciprocally drive the elements. When the pump assembly 40 isoperating, fluid exits the pumping elements 40 a, 40 b, and 40 c via anassociated pump outlet line 48 and is delivered to a location upstreamof the apply valves 24.

A rear brake subsystem, generally designated 53, may be coupled to thesecondary brake line 22 and may operate in a manner similar to thatdescribed above, but with fluid exiting the pumping elements 40 d, 40 e,and 40 f. In this manner two separated, isolated hydraulic systems orbrake circuits provide a front/rear split for brake redundancy in awell-known manner. However various other splits including diagonalsplits and the like may be provided.

The brake system 10 may further include a pair of normally closed primevalves 21 and a pair of normally open isolation valves 25. Each primevalve 21 is coupled to the associated brake line 20, 22 such that fluidcan flow from the master cylinder 12 to the pump assembly 40 via theassociated pump inlet line 42. Each isolation valve 25 is coupled to theassociated brake line 20, 22 to allow or block the flow of fluid betweenthe master cylinder 12 and the associated apply valves 24 and wheelbrake subsystems 31.

The brake system 10 may also further include a pair of isolation checkvalves 27, with each isolation check valve 27 being in parallel with theassociated isolation valve 25; a pair of prime check valves 29, witheach prime check valve 29 being in parallel with the associated primevalve 21; and a pair of pump accumulator check valves 33, with each pumpaccumulator check valve 33 being disposed between one of theaccumulators 36 and the input of the associated pumping elements 40 a,40 b, and 40 c or 40 d, 40 e, and 40 f.

The brake system 10 may additionally include a plurality of sensors tomonitor the status of the vehicle. In particular, the brake system 10may include wheel speed sensors 54, a brake pedal position sensor 56,and a fluid level sensor 60 to measure fluid levels in the reservoir 14.The system may further include a yaw rate sensor 50, a steering wheelangle sensor 55, a lateral acceleration sensor 57, a longitudinalacceleration sensor 59, and pressure sensors 61. Each of the sensors 50,54, 55, 56, 57, 58, 59, 60, 61 may be operatively coupled to anelectronic control unit or “ECU” 62 which can receive and/or processinputs from the various sensors. The ECU 62 may also be operativelycoupled to each of the apply 24 and release 26 valves, the pump motor46, the prime valves 21, the isolation valves 25, and the enginethrottle 58 to control and monitor these components.

The system 10 illustrated in FIG. 1 provides electronic stabilitycontrol (“ESC”), which may also be referred to as vehicle stabilityenhancement (“VSE”) or an electronic stability program (“ESP”). ESC isan electromechanical control system designed to monitor and influencewheel dynamics, and ultimately vehicle dynamics, during a vehicle stateof braking, steering, accelerating, or coasting. ESC typically usesinput from wheel speed sensors 54, steering wheel angle sensor 55, yawrate sensor 50, lateral acceleration sensor 57, and, optionally,longitudinal acceleration sensor 59 to determine both the driver'sintended heading and the vehicle's actual heading. ESC typicallycontrols the application of a wheel brake subsystem 31 on a single wheelor two wheels simultaneously, as necessary, to help a driver regaincontrol in a skid caused by oversteering or understeering in a curve,but also may also operate to provide control and vehicle guidance invarious other manners.

During a controlled braking event (whether triggered by ABS, TCS, ESC,or the like), the ECU 62 operates the pump assembly 40 and the apply 24and release 26 valves to control the brake pressure applied to the wheelbrake subsystems 31 in a well-known manner. It should be understood thatthe brake schematic shown in FIG. 1 is provided as merely an example ofa single type of brake system in which the pump assembly 40/pumpingelements 40 a, 40 b, 40 c, 40 d, 40 e, and 40 f may be employed.

For example, during ABS control the apply 24 and release 26 valves areoperated to control the brake pressure applied to the wheel brakesubsystems 31 so that the applied pressure matches, as closely aspossible, the pressure requested by the driver while regulating wheelslip to provide the maximum brake torque available for a tire/roadinterface. Thus the apply 24 and release 26 valves, as well as the pumpassembly 40, can be operated by the ECU 62 to control braking pressurein the well-known manner of ABS control.

For further example, during TCS control it is generally desired to applybrake pressure to an excessively spinning wheel to cause torque totransfer to the other wheel on the same axle in a well-known manner.This type of pressure build mode requires brake fluid to flow from themaster cylinder 12 to the appropriate apply valves 24 and associatedwheel brake subsystems 31 without any user input. Thus the apply 24 andrelease 26 valves, pump assembly 40, prime valves 21, and isolationvalves 25 can be operated by the ECU 62 to control braking pressure inthe well-known manner of TCS control. Of course any of a variety ofvehicle brake systems may be utilized to implement ABS, TCS, and/or ESC,and FIG. 1 is merely illustrative of one such system.

FIG. 2 illustrates one aspect of a pumping element 40 a-f in the form ofa straight bore configuration. In particular, the pumping element 40 a-fincludes an outer sleeve 68 having an inlet 70 coupled to the pump inletline 42 and an outlet 74 coupled to the pump outlet line 48. A piston 76is slidably yet sealingly received within the outer sleeve 68. A filter(not shown) may be provided at the inlet 70. The piston 76 defines agenerally sealed primary pump cavity 78 located between the leadingsurface 80 of the piston 76 and the outer sleeve 68.

The piston 76 includes a central bore 82 including a generallyradially-extending portion 82 a communicating with the inlet 70 and agenerally axially extending portion 82 b leading to the leading surface80 of the piston 76. An inlet check valve, generally designated 84,which may be in the form of a ball valve, seats adjacent to the axiallyextending portion 82 b of the central bore 82. The inlet check valve 84is spring biased into the closed position by a spring 86 located betweena retainer 88 and the piston 76. The pumping element 40 a-f furtherincludes a throat 90 located adjacent to the primary pump cavity 78. Anoutlet check valve, generally designated 92, which may be in the form ofa ball valve, seats against the throat 90. The outlet check valve 92 isspring biased into a closed position by an outlet check valve spring 94.

During operation of the pumping element 40 a-f the piston 76 maycommence pumping operations when the piston 76 is in a top dead centerposition (i.e., moved fully to the left in FIG. 2). As the piston 76moves through a suction stroke (i.e., moves to the right) the size ofthe primary pump cavity 78 increases, thereby creating a relativesuction and drawing fluid in through the inlet 70 and into the centralbore 82 of the piston 76. The pressure differential between the primarypump cavity 78 and the central bore 82 causes the inlet check valve 84to open (i.e., the inlet check valve ball moves to the left from theposition shown in FIG. 2) to allow fluid to flow into the primary pumpcavity 78.

Once the piston 76 has moved to a bottom dead center position (i.e.,moved fully to the right in FIG. 2) the piston 76 begins to move througha discharge stoke (i.e., moves to the left). This discharge strokecompresses the fluid in the primary pump cavity 78 and creates anincreased pressure therein. The increased pressure opens the outletcheck valve 92 (i.e., the outlet check valve ball moves to the left fromthe position shown in FIG. 2) and pushes pressurized fluid through thethroat 90 and to the outlet 74 for use in the brake system 10. Thepiston 76 may continue to reciprocate in this manner to providepressurized brake fluid to the brake system 10 as needed.

FIG. 3 illustrates another aspect of a pumping element 40 a-f in theform of a stepped bore configuration. Piston 76′ is similar to piston76, but includes a relatively narrow trailing neck portion 96 such thata secondary pump cavity 98 is located between the radially-extendingbore portion 82 a of the piston 76′ and the outer sleeve 68. The piston76′ may be shaped to provide a 2:1 ratio such that the surface area onthe leading surface 80 of the piston head is about double the surfacearea on the opposite, stepped side 100 of the piston head. The secondarypump cavity 98 operates in an offset manner as compared to the primarypump cavity 78. In particular, as the piston 76′ of FIG. 3 moves fromthe top dead center position to the bottom dead center position (i.e.,moves left-to-right in FIG. 3) the primary pump cavity 78 grows largerand creates a suction force. Simultaneously the secondary pump cavity 98grows smaller and creates a temporary positive pressure at the inlet 70of the pumping element 40 a-f which improves the probability of fluidflowing through the inlet check valve 84 and into the expanding primarypump cavity 78. In this manner the secondary pump cavity 98 essentially“primes” the primary pump cavity 78 to ensure that, during the dischargestroke, the pumping element 40 a-f displaces an increased amount offluid and thus operates with overall greater efficiency.

When the piston 76′ moves from the bottom dead center position to thetop dead center position (i.e., moves right-to-left in FIG. 3) theprimary pump cavity 78 grows smaller and pushes pressurized fluidthrough the outlet check valve 92 and outlet 74. Simultaneously, thesecondary pump cavity 98 grows larger and thereby draws fluid in via theinlet 70. The stepped bore pumping element 40 a-f of FIG. 3 is sometimestermed a “two-stage” pump due to the dual stage pumping action. However,it is noted that this design is not necessarily a pure two-stage pump,and may instead be more properly classified as a modified single-stagepump or a stepped-bore pump.

Considering either the front 23 or rear 53 hydraulic circuit separately,FIG. 4 illustrates the theoretical pump output flow rates for a onepumping element (providing one pumping cycle per shaft revolution), twopumping element (providing two pumping cycles per shaft revolution, andoperatively spaced 180° apart), and three pumping element (providingthree pumping cycles per shaft revolution, and operatively spaced 120°apart) assembly based upon the instantaneous rate of shaft displacement.Operatively spaced apart, as used herein, means that the pistons of therelevant pumping elements will be at a bottom dead center position atdifferent points in a shaft revolution (e.g., 0° and 180°, or 0° and120° and 240° for the scenarios described above).

For a single pumping element, illustrated as the curve at far left ofFIG. 4, the bottom dead center position corresponds to 0° (and 360°).The fluid output flow is delivered in less than one-half revolution ofthe pump motor shaft with a corresponding significant rise in linepressure due to the rapid change in flow as the pumping element changesfrom no flow to full flow and back to no flow for each discharge stroke.In a controlled brake application this makes the brake pedal feel harshand also contributes to excess vibration and noise. Two pumping elementsoperatively spaced 180° apart, illustrated as the middle curve of FIG.4, provide up to twice the total flow rate. However the flow rateprofiles and resulting pressure pulses still produce unacceptable “NHV”performance (noise, harshness, and vibration performance) for the brakeuser even with the combined pumping elements as the instantaneous flowcan still vary from zero to full flow. Three pumping elementsoperatively spaced 120° apart, illustrated as the curve at far right ofFIG. 4, provide up to three times the total flow rate. Moreover sincethe output flows of the pumping elements now overlap, the totalinstantaneous flow becomes much more constant and never approaches zero.The number of pulses is doubled from the overlapping output flows, andthe total flow rate remains much more constant with the magnitude of thepulses being significantly reduced. As a result NHV performance is muchimproved.

From the type of analysis demonstrated above, it is believed that aneven number of pumping elements (e.g., 2, 4, etc) would not generate theout-of-phase flow discharges and the desirable improvements in NHVperformance. It is also believed that a higher number of pumping units(e.g., 5, 7, etc.) operating with an appropriate angular separationcould show marginal improvement in NHV performance, however it isbelieved that providing more than three pumping elements would not beeconomically justified.

Typically, brake systems including fewer pumping elements (i.e., thesingle element and two element scenarios discussed earlier in thecontext of FIG. 4) incorporate a damping chamber onto the pump outletline 48 consisting of a compliant member and a damper orifice in orderto dampen the fluid flow and reduce pressure fluctuations. However forthe brake systems illustrated in FIGS. 5 and 6 the amplitude of pressurepulsations of fluid being returned to the primary brake line 20 is smallwhen utilizing a 120° operative spacing in pumping element operation,and the damping chamber component may be eliminated from the designthereby providing a cost savings.

The horizontally opposed (i.e., boxer-style) hydraulic block shown inFIG. 5 is believed to be much less expensive to manufacture as comparedto the radial-style hydraulic block shown in FIG. 6. The boxer-stylearrangement utilizes a much simplified machining and assembly sequencethat reduces the number of interconnecting piston bores (i.e., the bores63 a, 63 b, 63 c that house the pumping elements 40 a-f, as shown inFIG. 7) required through the modulator body or hydraulic block. A brakesystem incorporating six stepped bore piston pump elements in aboxer-style arrangement also allows all orthogonal drilling andassembly, which further reduces cost by minimizing the number ofmounting fixtures required for both machining and assembly. Theboxer-style, six pump layout shown in FIG. 5 requires threeinterconnecting bores that may be orthogonally drilled through thehydraulic block. The radial-style, six pump layout as shown in FIG. 6requires six bores that are not axially aligned and therefore notorthogonally drillable.

In one aspect of the disclosure, shown in FIG. 7, six pumping elements40 a, 40 b, 40 c, 40 d, 40 e, and 40 f may be driven by a rotatableeccentric assembly 112 having three eccentric elements 112 a, 112 b, and112 c disposed with 120° angular spacing on a motor shaft 110 to providesignificantly improved NHV performance. The pumping elements 40 a, 40 b,40 c, 40 d, 40 e, and 40 f may include a stepped-bore configuration, ormay be a straight bore configuration, or may be either configurationpiston pump elements.

FIG. 8 illustrates press-fit eccentric needle bearing elements 112 a,112 b, and 112 c, which are commercially available from suppliers suchas INA Bearing Company. As shown in FIGS. 8A and 8B, each eccentricelement 112 a, 112 b, 112 c includes a solid outer ring 114, a rotatableinner ring 116, bearing cages 118 made from plastic or steel, and needlebearings 120 made from steel. The eccentric elements 112 a, 112 b, and112 c are press-fit onto the motor shaft 110 as illustrated in FIG. 8;but a wide variety of fastener designs may be utilized, such as but notlimited to eccentric locking collar fasteners, grub screw fasteners inthe inner ring, adapter sleeve fasteners, drive slot fasteners, or thelike. Each outer ring 114 may have a crowned or non-crowned outsidesurface. It is believed that press-fit eccentric elements 112 a, 112 b,and 112 c disposed with 120° angular spacing on a straight motor shaft110 provide superior NHV performance because the assembly facilitatesthe pumping system shown in FIG. 7. Alternatively, the shaft may beconstructed with more conventional ground-in eccentrics, but thisalternative design may impose assembly and space restrictions and bemore costly to manufacture.

Returning to FIG. 7, a first pair of pumping elements 40 a and 40 f maybe disposed within a first pump bore 63 a, a second pair of pumpingelements 40 b and 40 e may be disposed within a second pump bore 63 b,and a third pair of pumping elements 40 c and 40 d may be disposedwithin a third pump bore 63 c. Each pump bore 63 a, 63 b, 63 c issurrounded by a hydraulic block comprising a major portion of ahydraulic control unit body or “HCU” body 65. The pump layout furtherincludes a piston driver in the form of a rotatable eccentric assembly112 which is driven by the motor shaft 110 and used to drive a pair ofopposed pistons 76 or 76′ in their respective outer sleeves 68 asdescribed earlier. The motor shaft 110 may be supported by at least oneshaft bearing 111 in combination with at least one pilot bearing 113.The shaft bearing 111 and the pilot bearing 113 are received intojournals which provide deflection resistance (i.e., the ability tomaintain the theoretical longitudinal direction between the shaftbearing 111 and the pilot bearing 113) to the motor shaft 110, therebyimproving pump efficiency. The pilot bearing 113 can be press-fit ontothe outermost end of the motor shaft 110, and the rotatable eccentricassembly 112 inserted into a drive shaft bore 108 extending through thehydraulic block and intersecting the piston bores 63 a, 63 b, 63 c. Thehorizontally opposed pairs of pumping elements may be offset by 180°such that when the piston 76 or 76′ of one of the pumping elements(e.g., 40 a) is in the bottom dead center position the piston 76 or 76′of the other pumping element (e.g., 40 f) is in the top dead centerposition, although the pistons may be offset by various other degrees ormay not be offset at all.

When non-aligned pumping elements (e.g., 40 c and 40 f) are fullypositioned against their respective eccentric elements (e.g., 112 a and112 c) radial forces are exerted from the pumping elements onto theeccentric bearings during their respective discharge strokes and tend tobalance each other. For three pump units and eccentric elements having a120° angular spacing, a third pumping element (e.g., 40 e) maysimultaneously exert radial forces onto the third eccentric element(e.g., 112 b). Any unbalanced radial force resulting from the additiveeffects of these three radial forces may be offset by the journal/pilotbearing 113 in combination with the journal/shaft bearing 111, therebyavoiding or minimizing any deflection of the motor shaft 110.

A piston-biasing spring or piston return spring 87 may be located ineach primary pump cavity 78 to force the distal end of the piston 76 or76′ to abut a rotating eccentric element 112 a-c as shown in FIGS. 14and 15. However providing a piston return spring 87 in a primary pumpcavity 78 naturally displaces some brake fluid and reduces the pumpefficiency. Therefore external pump piston retainers 140 as shown inFIG. 13 may be employed to improve pump efficiency. Such a retainer 140may be located outside the pump cavity, adjacent to an eccentric element112 a-c and motor shaft 110 within the drive shaft bore 108, and beconnected to the distal ends of a pair of horizontally opposed pistons76 or 76′. The retainer 140 may be generally “C” shaped in end view andpulled into tension when fit around an eccentric element 112 a-c. Inthis manner the retainer 140 forces the pistons 76 or 76′ to abut theouter surface of the rotatable eccentric assembly 112 to ensure thatrotating motion of the assembly 112 is translated into a reciprocatingmotion of the pistons. The pump efficiency can be higher with theretainers 140 as no brake fluid is displaced from the primary pumpcavity 78 with the result that pump compression ratios are substantiallyimproved. However, providing a retainer 140 fit around each eccentricelement 112 a-c may increase installation costs. Therefore, for certainaspects of the disclosure, internal return springs 87 may be used eventhough some pump performance robustness may be sacrificed.

In a conventional hydraulic pump, a piston made from a hard material(e.g., steel) that is inserted directly inside a piston bore made from asoft material (e.g., aluminum alloy) would cause severe erosion of thebore as the outer surface of the piston slides and rubs against itsinner surface. Accordingly, a piston guide made from a very hardmaterial may be inserted to shield the piston bore. For example, apiston sleeve 68 made from a hard material (i.e., a full sleeve design)as illustrated in FIGS. 2 and 3 could support a hard material piston 76or 76′, collectively designated 77, operating inside a piston bore 63a-c. A body 65 made from a soft material surrounds the sleeve 68. A body65 constructed from a soft and light material, such as an aluminumalloy, may provide a significant benefit due to the highly desirableweight reduction when compared to a body 65 constructed from a materialsuch as steel.

As illustrated in FIG. 9, a pumping element 40 a-f may instead have apartial sleeve 116 enabling improved flow through inlet 70 and an inletfilter, if provided. Cost reduction attributable to a partial sleeve116, sometimes termed “half-sleeve,” may be realized by eliminating thedistal sleeve portion (i.e., in proximity to the inlet 70) and retainingthe proximal sleeve portion of the full sleeve 68 shown in FIG. 3. Withthe half sleeve 116, O-rings may be used to seal to the surrounding bodyas shown, or alternatively, an interference fit or “press fit” may beused as shown in FIG. 7. The piston 77 may be a one-piece, high-heat,wear resistant polymer piston 77 including a small press-on steel cap114 which functions as an eccentric wear surface. The half sleeve designis believed to be less expensive than designs incorporating a fullsleeve 68, a separate external outlet check valve assembly, a steelpiston or composite steel/plastic piston design, and/or a separate pumpbore cap.

In one aspect of the disclosure, illustrated in FIG. 14, the pumpingelement 40 a-f includes pistons 77 manufactured from a polymericmaterial in combination with a partial sleeve 116 (i.e., “half-sleeve”design) to protect a relatively soft housing cavity 150. FIG. 14 showsthe pumping element shown in FIG. 9 installed within a housing cavity150. A low-pressure seal 143 may be mounted around the circumference ofthe piston 77 on a distal portion of the piston 77 near the wear cap114. It should be noted that, as illustrated in FIG. 14, an O-ring seal144 may be used as the low-pressure seal 143 if desired, to facilitatethe removability and inspection of the piston 77, but other elastomericseals may be employed. A high-pressure seal 145 includes the interfacesurface between a circumference of the proximal portion of the piston 77and the partial sleeve 116.

As illustrated in FIG. 10, a pumping element 40 a-f may also insteadhave a sleeveless design incorporating a one-piece polymeric piston 77with a press-on wear cap 114. The piston 77 may employ a stepped boreconfiguration or may be a straight bore configuration or may be eitherconfiguration and be used in the manufacture of a simplified, low costbrake pump assembly. A sleeveless design including a check valve housing92, a pump bore cap 120, and a high-heat, wear resistant polymer piston77 is believed to be less expensive than designs incorporating aseparate external outlet check valve assembly, a steel piston or othercomposite steel/plastic piston designs, separate full sleeve orhalf-sleeve, and/or separate pump bore cap. The sleeveless designenables minimized bore-to-bore spacing (i.e., no piston sleeve isrequired) and minimized motor shaft length, thereby avoiding additionalexpense.

In one aspect of the disclosure, as illustrated in FIG. 15, a piston 77manufactured from a softer, polymeric material (i.e., sleeveless design)may be used instead of a hard material piston sleeve (i.e., full sleeveor half-sleeve design) to protect a relatively soft housing cavity 150.FIG. 15 shows a pumping element having the design illustrated in FIG. 10installed within a housing cavity 150. A low-pressure seal 147 may bemounted around the circumference of the piston 77 on a distal portion ofthe piston 77 near the wear cap 114. It should be noted that, asillustrated in FIG. 15, an O-ring seal 148 may be used as thelow-pressure seal 147 if desired, but other elastomeric seals may beemployed. A high-pressure seal 149 includes the interface surfacebetween a circumference of the proximal portion of the piston 77 and thehousing cavity 150. It should be noted the piston bore 63 a-c requires ahighly polished finish to facilitate the high-pressure seal 149described above. The smooth surface finish required for the piston 77may be obtained by using highly polished tooling during fabrication byinjection molding.

A polymer piston may also be employed to simplify other aspects of brakepump assembly manufacturing. For example, during ABS control eventsbrake fluid released into the accumulators 36 may flow to the pumpingelements 40 a-f under very high pressure, such as when braking hard on adry tire/road interface. Such pressures in combination with thereciprocating motion of the pistons 77 may cause significant volumes ofbrake fluid to leak past the low-pressure seals 143 or 147 and into thedrive shaft bore 108 of a brake pump assembly. If sufficient brake fluidwere to build up within the drive shaft bore 108 then it might foul themotor 46 driving the brake pump assembly, causing a partial failure ofthe vehicle braking system. To prevent such failures brake pumpassemblies typically include a drain line in fluid communication withthe drive shaft bore, and may include a combination of sponges and/orseals disposed about the means for driving the pistons to isolate themotor from the rest of the assembly. However these fluid controlelements increase the cost and part count of the brake pump assembly, aswell as the complexity of the assembly sequence. To reduce the volume ofleaked brake fluid to be controlled the pumping elements could bemodified to include multiple low-pressure seals, but such designs becomesusceptible to a condition known as “hydraulic lock,” where pressurizedfluid trapped between multiple seals generates a resistance force thatmay immobilize a piston. In the aspects discussed above such aresistance force might exceed the return force generated by a pistonreturn spring 87 or external pump piston retainer 140, cutting short orpossibly eliminating a piston's suction stroke. A polymer piston 77 mayinstead provide a directional seal 81 that provides the benefit ofmultiple seals but is not susceptible to “hydraulic lock,” thuspermitting the simplification of, minimization of, or possibleelimination of the drive shaft bore fluid control elements. It isbelieved that a polymer piston providing a directional seal reduces unitcosts because the part count and complexity of the assembly sequence arereduced, particularly in multiple assembly devices such as the sixelement brake pump assembly discussed above.

In one aspect of the disclosure, as illustrated in FIG. 16, a pumpingelement 40 a-f may have a straight bore configuration incorporating apolymeric piston 77, and a portion of the polymeric piston 77 may serveas both a high-pressure seal 149 and a directional low-pressure seal toreduce leakage into the drive shaft bore 108. The piston 77 includes aperipheral sealing lip 81 a surrounding the perimeter of the leadingsurface 80. The peripheral sealing lip 81 a projects primarilylongitudinally away from the leading surface 80 toward the pump cavity78, and is molded, machined, or otherwise treated to project partiallyradially outward from the leading surface 80 toward the side walls ofthe housing cavity 150/pump bore 63 a-c. The peripheral sealing lip 81 apreferably projects outward from the side surface of the piston at anangle of about 5 degrees to about 10 degrees with respect to thelongitudinal axis of the piston 77 prior to installation, and may havean average thickness of between about 0.1 mm and about 0.3 mm. Theoutward projection of the peripheral sealing lip 81 a past the sidesurface of the piston 77 produces an interference fit with the housingcavity 150, but the outward angle of the lip, the thickness and profileof the lip, and the elasticity of the piston polymer material arepreferably selected so that the peripheral sealing lip 81 a may flexaway from the side walls of the housing cavity 150 in response to apressure differential of about 0.5 bars to about 2 bars between thedistal and proximal sides of the seal. The longitudinal projection ofthe peripheral sealing lip 81 a past the leading surface 80 permits thefluid pressure in the pump cavity 78 during a discharge stroke toreinforce the biased sealing engagement of the lip 81 a against the sidewalls of the housing cavity 150, so that the high-pressure seal 149 inthis aspect constitutes the interface surface between the peripheralsealing lip 81 a and the housing cavity 150.

The leading surface 80 of the piston may optionally be contoured todefine an annular channel 79 a and projection 79 b for receiving andcentering a return spring 87. Preferably, a distal portion of theperipheral sealing lip 81 a defines a portion of the annular channel 79a, and the lip 81 a is trapezoidal such that the proximal end of theperipheral sealing lip 81 a is spaced apart from the return spring 87 tobe received in the channel 79 a. Alternately, the leading surface of thepiston may be substantially flat or configured to have any otherdesirable profile when an external pump piston retainer 140 is employed.The distal end of the piston 77 may also optionally include a wear cap114 (not shown), or be configured to engage an external pump pistonretainer 140 in manners similar to that illustrated in FIG. 7 (also notshown).

A low-pressure seal 147 is mounted around the circumference of thepiston 77 behind the high-pressure seal 149. An O-ring seal 148 may beused as the low-pressure seal 147, if desired, to facilitate theremovability and inspection of the piston 77, but other elastomericseals may be employed in its stead. The side surface of the piston 77and the side walls of the housing cavity 150 between the low-pressureseal 147 and the high-pressure seal 149 define a nominally closedclearance gap 151, such as that which would be found in a solid pistonhaving multiple O-ring seals. The low-pressure seal 147 is preferablymounted over a medial portion of the piston 77 so that it does notencounter the bearing wear path of the piston, which in the illustratedaspect would constitute that portion of the housing cavity 150 engagedby the peripheral sealing lip 81 a as well as that portion of thehousing cavity 150 engaged by the distal end of the piston 77. Mostmeans for driving the piston 77, such as the rotatable eccentricassembly 112 described above, will exert a lateral force against it,causing the piston 77 to cant about the low-pressure seal 147 so that aportion of the distal end and an opposite portion of the leading surface80/peripheral sealing lip 81 a bear against the side walls of thehousing cavity 150. Any debris that migrates between the piston 77 andthe side walls of the housing cavity 150 may be dragged across thesebearing surfaces and eventually deposited at the ends of the wearpath(s). If the typically very soft low-pressure seal 147 shouldreciprocate over the accumulated debris or a scored side wall surface itwill tend wear comparatively quickly, and potentially cause an excessiveleakage of brake fluid into the drive shaft bore 108 of the device. Bypositioning the low-pressure seal 147 over a medial portion of thepiston 77, between the bearing wear paths, wear due to abrasion can beminimized and the durability of the pumping element 40 a-f can be eithermaintained or increased.

In comparison to the pistons 77 shown in FIGS. 9 and 10, the piston ofthis aspect includes a peripheral sealing lip 81 a that acts as both ahigh-pressure seal 149 and an additional, directional seal to resistleakage into the drive shaft bore 108 during a pump suction stroke. Theperipheral sealing lip 81 a is directional in that it may selectivelydisengage from the side walls of the housing cavity 150 during a pumpsuction stroke in response to an accumulation of pressurized brake fluidbehind the distal side of the seal, thereby preventing a build up ofpressure sufficient to cause a hydraulic lock condition. Since both thehigh-pressure seal 149/directional seal 81 a and low-pressure seal 147are intended to act in concert during a pump suction stroke, theexemplary piston 77 does not include a central bore 82, the pumpingelement inlet 70 is instead positioned ahead of the leading surface 80of the piston 77 at top dead center position, and the inlet check valve84 is instead mounted within or in fluid communication with the inlet70.

During operation of the pumping element 40 a-f the piston 77 maycommence pumping operations when the piston 77 is at bottom dead centerposition. Peripheral sealing lip 81 a is biased into sealing engagementwith the side walls of the housing cavity 150 to provide a high-pressureseal 149 in the pumping element. As the piston 77 is driven toward topdead center position, brake fluid is compressed within the pump cavity78 and the increased pressure opens the outlet check valve 92, pushingthe fluid through the check valve and outlet 74 (not shown) for use inthe brake system 10. The increased pressure also creates a negativepressure differential between the clearance gap 151 and pump cavity 78,causing the brake fluid in the pump cavity 78 to further press theperipheral sealing lip 81 a into sealing engagement with the side wallsof the housing cavity 150. The increased pressure in combination withthe motion of the piston 77 may still cause some brake fluid to migratepast the high-pressure seal 149 and accumulate within the clearance gap151, however the directional nature of the seal will prevent a hydrauliclock condition, as discussed below.

When the piston reaches top dead center position and begins to be driventoward bottom dead center position, pressure within the pump cavity 78decreases to produce a relative suction, and brake fluid in theaccumulator 36 may be driven into or drawn into the pump cavity 78through the inlet 70. If pressurized brake fluid has built up within theclearance gap 151 and there is a positive pressure differential betweenit and the brake fluid within the pump cavity 78, i.e., brake fluid inthe clearance gap 151 has developed a greater pressure than the brakefluid entering the pumping element, then the peripheral sealing lip 81 amay flex away from the side walls of the housing cavity 150 todepressurize the fluid within the clearance gap 151, causing thepressure within the clearance gap to approximate the inlet fluidpressure. On the other hand, if there is no significant pressuredifferential or a negative pressure differential then the peripheralsealing lip 81 a will remain in sealing engagement with the side wallsof the housing cavity 150 to both maintain that differential and toserve as an additional seal against leakage into the drive shaft bore108.

Those skilled in the art will recognize that the pressure of the brakefluid within the clearance gap 151 will initially be lower than thepressure of any brake fluid driven from an accumulator 36 during acontrolled braking event, and only slowly build towards higher pressuresduring the course of the event. Advantageously, the pressure of thebrake fluid within the clearance gap 151 may be reduced to a baselinelevel by simply manipulating the brake system valves and running thebrake pump assembly 40 for a short time after the conclusion of acontrolled braking event. The peripheral sealing lip 81 a permits thefluid pressures within the clearance gap 151 to be moderated to reduceleakage through the low-pressure seal 149 while eliminating thepotential for entrapping fluid pressurized to such an extent as to causea hydraulic lock condition.

In another aspect of the disclosure, as illustrated in FIG. 17, apumping element 40 a-f may have a stepped-bore configurationincorporating a polymeric piston 77 having a high pressure seal 149 anda separate directional low-pressure seal to reduce leakage into thedrive shaft bore 108. The directional low-pressure seal is preferablyprovided in addition to an elastomeric low-pressure seal, such as anO-ring seal, because it is believed that the latter seal is better ableto resist “static leakage” when the brake pump assembly is idle. Howeveran elastomeric low-pressure seal is not a necessary element of thedevice, which could, for example, include a plurality of directionallow-pressure seals.

In an exemplary piston 77 similar to that shown in FIG. 15 and discussedearlier, the high-pressure seal 149 includes the interface surfacebetween a circumference of the proximal portion of the piston 77 and thehousing cavity 150, and a low-pressure seal 147 is mounted around thecircumference of the piston 77 on a medial portion adjacent theeccentric-engaging end. An O-ring seal 148 may be used as thelow-pressure seal 147 if desired, but other elastomeric seals may beemployed in its stead.

The piston 77 includes a peripheral sealing lip 81 b surrounding aperimeter of the trailing neck portion 96 proximal to the low-pressureseal 147. The peripheral sealing lip 81 b projects primarilylongitudinally toward the secondary pump cavity 98 of the pumpingassembly 40 a-f, and is molded, machined, or otherwise treated toproject partially radially outward from the trailing neck portion 96toward the side walls of the housing cavity 150/pump bore 63 a-c. Theperipheral sealing lip 81 b preferably projects outward from the sidesurface of the trailing neck portion 96 at an angle of about 5 degreesto about 10 degrees with respect to the longitudinal axis of the piston77 prior to installation, and may have an average thickness of betweenabout 0.1 mm and about 0.3 mm. The outward projection of the peripheralsealing lip 81 b past the trailing neck portion 96 of the piston 77produces an interference fit with the housing cavity 150 behind thesecondary pump cavity 98, providing an additional low-pressure seal. Theside surface of the trailing neck portion 96 and the side walls of thehousing cavity 150 between the low-pressure seal 147 and the peripheralsealing lip 81 b define a nominally closed clearance gap 151, such asthat which would be found in a piston having stacked O-ring seals, butthe outward angle of the lip, the thickness and profile of the lip, andthe elasticity of the piston polymer material are preferably selected sothat the peripheral sealing lip 81 b may flex away from the side wallsof the housing cavity 150 in response to a pressure differential ofabout ? bars to about ? bars between the distal and proximal sides ofthe seal. The longitudinal projection of the sealing lip 81 b past thetrailing neck portion 96 produces a seal that may be further pressedagainst the side walls of the housing cavity 150 by pressurized fluid inthe secondary pump cavity 98, but flexed away from the side walls of thehousing cavity 150 as the piston 77 reciprocates to depressurize thefluid within the clearance gap 151.

During operation of the pumping element 40 a-f, the piston 77 maycommence pumping operations when the piston 77 is at top dead centerposition. Peripheral sealing lip 81 b is biased into sealing engagementwith the side walls of the housing cavity 150 to provide an additionallow pressure seal in the pumping element. As the piston 77 is driventoward bottom dead center position, the primary pump cavity 78 growslarger and creates a suction force while simultaneously the secondarypump cavity 98 grows smaller and creates a temporary positive pressureso as to prime the primary pump cavity 78. When the piston 77 reachesthe end of its cycle, reverses, and is driven toward top dead centerposition, the primary pump cavity 78 grows smaller and pushespressurized fluid through the outlet check valve 92 and outlet 74 whilesimultaneously the secondary pump cavity 98 grows larger and draws fluidinto the secondary pump cavity 98 via the inlet 70. During eithermotion, brake fluid released into the accumulators 36 during acontrolled braking event may flow into the secondary pump cavity 98under very high pressure, potentially subjecting the low-pressure sealsto pressures sufficient to create a hydraulic lock condition betweenstacked O-ring type seals. However, if pressurized brake fluid has builtup within the clearance gap 151 and there is a positive pressuredifferential between it and the brake fluid within the secondary pumpcavity 98 then the peripheral sealing lip 81 a may flex away from theside walls of the housing cavity 150 to depressurize the fluid withinthe clearance gap 151, causing the pressure within the clearance gap toapproximate the inlet fluid pressure. On the other hand, if there is nosignificant pressure differential or a negative pressure differentialthen the peripheral sealing lip 81 b will remain in sealing engagementwith the side walls of the housing cavity 150 to both maintain thatdifferential and to serve as an additional seal against leakage into thedrive shaft bore 108.

Those skilled in the art will recognize that the pressure of the brakefluid within the clearance gap 151 will generally be lower than thepressure of any brake fluid driven from an accumulator 36 during acontrolled braking event, and only slowly build towards higher pressuresduring the course of the event. Preferably, the outward angle of thelip, the thickness and profile of the lip, and the elasticity of thepiston polymer material are selected so that the peripheral sealing lip81 b does not flex away from the side walls of the housing cavity 150 inresponse to the pressure differential between a typical suction statepressure and the positive priming pressure temporarily created in thesecondary pumping cavity 98 so as to prevent premature wear and/orfatigue of the peripheral sealing lip feature.

High-heat, wear resistant engineering thermoplastics may be used toreplace traditional metal parts in many of the components disclosedherein to reduce weight and cost per part. These polymers may beinjection molded to form a wide variety of parts, such as but notlimited to gears, bearings, pistons and other brake pump assemblycomponents. Representative examples of high-heat, wear-resistantpolymers which may be used include but are not limited to polyimide(manufactured under the trademark KAPTON® by the DuPont Company),polyetherimide (manufactured under the trademark ULTEM® by the GeneralElectric Company), polyether ether ketone (manufactured under thetrademark PEEK™ by Victrex PLC), and the like. More particularly thepolymeric pistons 77 described herein can be made of fully or partiallyof these polymers. These polymeric materials are lighter and havegreater wear resistance and temperature resistance than many steel-basedmetals, which translates into a longer piston service life, as well asless fuel consumption by the vehicle during the service life of thepolymeric piston 77.

In additional aspects of this disclosure, polyether ether ketone may beused to produce brake pump assemblies having lower unit costs withoutsignificantly detracting from performance and durability standards orNHV performance. PEEK™ is a semicrystalline material, which retains itsmechanical properties even at very high temperatures. Unfilled PEEK™ maybe characterized by one or more of the following properties: a meltingpoint of between about 340° C. and about 350° C.; a density of about 1.3gram/cm³; water absorption of less than about 0.5 weight percent; andtensile strength of about 14,500 psi (at 23° C.) and about 1,740 psi (at250° C.). Furthermore, PEEK™ has a low coefficient of friction andresists attack by a wide range of organic and inorganic chemicals, suchas but not limited to brake fluid. PEEK™ polymer is commonly mixed withother resins or fillers such as glass or carbon. As properties vary withfiller type and content, compounds can be formulated to meet specificend-use requirements.

In accordance with other aspects of the disclosure, as illustrated inFIGS. 11 and 12A-B, the HCU body 65 may be incorporated within or formedas part of an EHCU packaging design, generally designated 135.Furthermore, the EHCU 135 may be adapted to include an embedded wheelbrake subsystem pressure sensor 130 (see FIGS. 11 and 12B) which sensesbrake fluid pressure in one or more of the wheel brake subsystems 31.Such an embedded sensor eliminates additional installation steps as wellas most external sensor wiring, and the pressure information provided bythe pressure sensor 130 may be required by the ECU 135 to perform thevarious brake system control tasks described above. Thus, a brakingsystem 10 is disclosed which, compared to prior art braking devices,incorporates an embedded wheel brake subsystem pressure sensor 130without departing from an objective of achieving a compact type ofconstruction and low manufacturing cost. An important aspect in thisrespect is the constructive extension of the functionality of an alreadyexisting highly integrated control mechanism, EHCU 135, by exploiting asmall mounting space available within the HCU body 65.

The EHCU 135 includes a pump motor 46, an ECU 125, and an HCU body 65disposed between the pump motor 46 and the ECU 125. The HCU body 65 mayinclude the elements schematically shown in FIG. 1, excluding wheelbrake subsystems 31, sensors 50, 55, 56, 57, 58, 59, 60, ECU 62, and themaster cylinder/reservoir 12, 14, and having the hydraulic andmechanical configuration shown in FIGS. 5 and 7. As described earlier,the HCU may thus contain pumping elements 40 a and 40 f; or 40 a, 40 b,40 e, and 40 f; or 40 a, 40 b, 40 c, 40 d, 40 e, and 40 f, etc. arrangedin a horizontally opposed (i.e., boxer-style) configuration. The ECU 125comprises the overall electronic system (not shown) of the EHCU 135,such as electromagnetic coils for actuating valves, electrical contacts,power semiconductors, microcontrollers, and the like, which aregenerally provided on a single circuit board. Current vehicle assemblyregulations may require the ECU 125 to be separable from the HCU body 65for servicing purposes, and reconnectable to the HCU body 65 thereafter.Thus, each apply valve 24, release valve 26, isolation valve 25, andprime valve 21 is preferably formed as a solenoid valve 134 switchablebetween open and closed positions by a solenoid coil subassembly (notshown). The plurality of solenoid valves 134 are preferably exposed atthe interface between the HCU body 65 and the ECU 125, and nest intointegral female connectors (not shown) on the ECU 125. Also, a motorconnector plug 133 may be provided to facilitate electricalcommunication between the ECU 125 and the motor 46, and nest into asocket connector (not shown) in the ECU 125.

Pressure sensors 61 shown in FIG. 1 may comprise wheel braking subsystempressure sensor(s) 130 and master cylinder pressure sensor 132. Wheelbraking subsystem pressure sensor(s) 130 typically cannot be mountedlike master cylinder pressure sensor 132 because they must sample fluidpressure downstream from an apply valve 24, in a region of the EHCUwhere hydraulic and mechanical components are optimally provided in anextremely dense packaging such as that shown in FIGS. 11 and 12B.However, an advantage of embedding a pressure sensor 130 within the HCUbody 65 is that the pressure sensor 130 is protected againstenvironmental influences to a much better degree than a sensor 61 thatis arranged externally. The pressure sensor(s) 130 as shown in thepresent figures advantageously may exploit a small mounting spaceavailable within the HCU body 65, wherein the sensor(s) 130 are embeddedin the HCU body 65 and located adjacent to but vertically separated fromthe horizontally opposed pumping elements 40 a-f. Such a space isevident in the cut-away view of a horizontally opposed hydraulic blockconfiguration shown in FIG. 5, but essentially absent in the cut-awayview of a radial-style hydraulic block configuration shown in FIG. 6. Asillustrated in the cut-away section of FIG. 11, the pressure sensor 130has two ends, with a hydraulic sampling end containing a pressuretransducer oriented toward valves 134 and ECU 125, and a signal endincluding a plurality of electrical pins typically oriented toward themotor unit 46. The electrical pins may provide an external electricalconnection for communication via connection ports 115 a and 115 b and anexternal harness device (not shown) in direct or indirect electricalcommunication with the ECU 125. The harness may take a variety of forms,such as but not limited to a portion of a vehicle wiring harness, aseparate jumper harness, etc.

Alternately, as illustrated in FIG. 12, connection ports 115 a and 115 bmay be omitted, and the electrical pins may instead be operativelyconnected to a lead frame 131 a and connector plug 131 b. The lead frame131 a and connector plug 131 b advantageously may further exploit asmall space available within the HCU body 65, wherein the connector plugportion 131 b is substantially embedded within the HCU body 65 andextending parallel to, but vertically separated from, the longitudinalaxis of the rotatable eccentric assembly 112, in a volume which isunlikely to be occupied by any hydraulic and/or mechanical portions ofthe brake pump assembly. Such a space is evident along the verticalcenterline of the cut-away view shown in FIG. 5. The lead frame 131 amay be received or receive a connector on the HCU body 65, andoperatively connected to the electrical pins of the pressure sensor(s)130. The connector plug 131 b may be slideably received within HCU body65, extending therethrough to ECU 125 to provide a substantially orwholly embedded signal path between pressure sensor(s) 130 and theelectronic system of ECU 125.

Having described the disclosure in detail and by reference to specificaspects thereof, it will be apparent that modifications and variationsare possible without departing from the spirit and scope of thedisclosure as defined by the following claims.

1. A brake pump assembly for pressurizing a fluid comprising: a blockdefining a piston bore, a piston slidably disposed in said piston boreincluding a trailing neck portion and defining a primary pump cavity onone side of said piston, a low pressure seal disposed between saidpiston and said piston bore, a first sealing lip integral with andprojecting longitudinally from said piston toward said primary pumpcavity and spaced from said low pressure seal and sealed to said pistonbore to define a secondary pump cavity between said trailing neckportion and said first sealing lip and for flexing radially inwardly inresponse to a positive pressure differential between the fluid in saidsecondary pump cavity and the fluid in said primary pump cavity to allowfluid to flow from said secondary pump cavity to said primary pumpcavity, said first sealing lip extending radially outwardly from saidpiston for biasing said sealing lip radially toward said piston bore inresponse to a negative pressure difference between the fluid in saidsecondary pump cavity and the fluid in said primary pump cavity, and asecond sealing lip integral with and projecting longitudinally from saidtrailing neck portion of said piston toward said secondary pump cavitywherein said second sealing lip extends radially outwardly from saidtrailing neck portion of said piston for sealing engagement with saidpiston bore.
 2. The brake pump assembly of claim 1, wherein said firstsealing lip engaging said piston bore forms a high-pressure seal torestrict fluid from flowing from said primary pump cavity into saidsecondary pump cavity.
 3. The brake pump assembly of claim 2, whereinsaid first sealing lip projects primarily longitudinally away from saidsecond sealing lip for sealing engagement with said piston bore.
 4. Thebrake pump assembly of claim 3 wherein said piston extends along alongitudinal axis, said piston has a generally cylindrical side surface,and said first sealing lip projects radially outward from said sidesurface at an angle of between 5 degrees and 10 degrees from thelongitudinal axis of said piston prior to insertion within said pistonbore.
 5. The brake pump assembly of claim 3 wherein said first sealinglip is configured to flex inwardly to allow fluid to flow from saidsecondary pump cavity to said primary pump cavity only in response to apositive pressure differential of 0.5 to and 2.0 bars between the fluidin said secondary pump cavity and the fluid in said primary pump cavity.6. The brake pump assembly of claim 3 wherein said low pressure seal ismounted around the circumference of said trailing neck portion of saidpiston.
 7. The brake pump assembly of claim 1, wherein said pistonextends along a longitudinal axis, said trailing neck portion has agenerally cylindrical side surface, and said leading surface projectsradially outward from said side surface at an angle of between 5 degreesand 10 degrees from the longitudinal axis of said piston prior toinsertion within said piston bore.
 8. The brake pump assembly of claim1, wherein: said trailing neck portion of said piston defines a narrowedmedial portion; and said low-pressure seal is a non-directional sealmounted around said medial portion such that said non-directional sealwill not encounter a bearing wear path of said piston during normalreciprocating operation of said piston.
 9. The brake pump assembly ofclaim 1 wherein said piston is formed of a polymeric material.
 10. Thebrake pump assembly of claim 9, wherein said polymeric material ischosen from the group consisting of polyimide, polyetherimide andpolyether ether ketone.
 11. The brake pump assembly of claim 10, whereina distal end of said piston includes a press-on steel wear cap.